Solders and soldering techniques are widely used in electronics manufacturing, including electronics assembly and packaging. Solder paste is one of the main forms of solder materials used in industry to bond and assemble electronic components of various types into electronic devices or photonic devices. One main usage of solder paste is through a ball grid array (BGA) technique for surface mounting packaging. Solder paste is composed of two materials: solder balls (normally in the micron size) and solvents that are used to prepare the paste, which usually contains a flux that is used to clean the surface oxide of the solder balls to ensure complete solder reflow upon melting. The metallic solder balls are evenly dispersed in a paste flux that allows printing through stencil apertures that match substrate pad locations, and reflowing the printed assembly at elevated temperatures which melts and ultimately forms the joints and interconnects.
Traditionally, the most widely used solder material is eutectic tin-lead (Sn—Pb) alloy (63/37 wt %). Due to the toxicity and health concerns of lead, lead-based solders are being phased out of the electronics manufacturing processes. There have been many lead-free solder candidates, and the most widely used candidates include tin/silver/copper (Sn/Ag/Cu, also referred to as SAC), Sn/Ag, and Sn/Cu. In certain processes or products, complete replacement with lead-free solders has been achieved. However, for certain industries such as defense, aerospace and medical devices, many lead-free candidates cannot achieve the same quality as Sn—Pb solders and may lead to reliability issue. Thus these industries are currently exempted from the lead-based solders. Also, for many lead-free solders such as SAC solders, the melting points are around 220° C. or above (depending on compositions), which may lead to higher processing temperatures during electronics manufacturing and thus increased thermal stress on the circuit board. Another significant issue for lead-free solder materials is the formation of tin whiskers, which may lead to short circuits for the electronics devices.
With the electronics industry's focus on further miniaturization and push to the “nanoelectronics” era, nanosolders are expected to play a key role in assembling nano-components (nano-building blocks) into nanoelectronics and nanosystems due to several orders smaller of solder size and potential lower melting temperature, for example, MEMS (Microelectromechanical systems) packaging and microBGA packaging.
One prior art paper by H. Jiang et al, titled “Synthesis and Thermal and Wetting Properties of Tin/Silver Alloy Nanoparticles for Low Melting Point Lead-Free Solders,” which was published in Chem. Mater. 2007, 19, 4482-4485, and appeared online on Aug. 11, 2007, is said to describe tin/silver alloy nanoparticles with various sizes that were synthesized via a low-temperature chemical reduction method, and their thermal properties were studied by differential scanning calorimetry. The particle size dependency of the melting temperature and the latent heat of fusion was observed. The melting point was achieved as low as 194° C. when the average diameter of the alloy nanoparticles was around 10 nm. The wetting test for as-prepared 64 nm (average diameter) SnAg alloy nanoparticle pastes on a Cu surface showed the typical Cu6Sn5 intermetallic compound (IMC) formation. These low melting point SnAg alloy nanoparticles could be used for low-temperature lead-free interconnect applications.
Also known in the prior art is Arvin et al., U.S. Patent Application Publication No. 2013/0284495 A1, published Oct. 31, 2013, which is said to disclose inert nano-sized particles having dimensions from 1 nm to 1,000 nm are added into a solder ball. The inert nano-sized particles may comprise metal oxides, metal nitrides, metal carbides, metal borides, etc. The inert nano-sized particles may be a single compound, or may be a metallic material having a coating of a different material. In another embodiment of the present invention, a small quantity of at least one elemental metal that forms stable high melting intermetallic compound with tin is added to a solder ball. The added at least one elemental metal forms precipitates of intermetallic compounds with tin, which are dispersed as fine particles in the solder.
Also known in the prior art is Zinn, U.S. Pat. No. 8,834,747, issued Sep. 16, 2014, which is said to disclose compositions containing tin nanoparticles and electrically conductive particles. The tin nanoparticles can have a size below about 25 nm so as to make the compositions fusable at temperatures below that of bulk tin (m.p.=232° C.). Particularly, when the tin nanoparticles are less than about 10 nm in size, the compositions can have a fusion temperature of less than about 200° C. The compositions can contain a whisker suppressant to inhibit or substantially minimize the formation of tin whiskers after tin nanoparticle fusion. In some embodiments, the compositions contain tin nanoparticles, electrically conductive particles comprising copper particles, and a whisker suppressant comprising nickel particles. Methods for using the present compositions are also described herein. The compositions can be used as a lead solder replacement that allows rework to be performed.
There is a need for improved lead-free solders having relatively low processing temperatures.